The cellular effects of carbon monoxide in the airway

The notion of inhaled carbon monoxide (CO) being a toxic chemical under all circumstances is currently being challenged, as recent research has suggested that low concentrations of CO may have therapeutic value, especially in the airway. This review evaluates CO's effects on cellular functions that may result in beneficial outcomes in the settings of airway disease, inflammation, and injury. CO can modulate the stress response system of the cell by decreasing levels of reactive intermediates over time, produced by mitochondrial iNOS and NADPH oxidase. Intracellular stress-induced response factors (e.g., HIF-1 and HSP- 70) are induced in response to CO, possibly facilitating more rapid and effective defenses, in response to subsequent stressors. CO also can trigger changes in cellular functions downstream, protecting the cells from stress-associated events promoted in the airway, as a result of disease or injury, including reducing rates of apoptosis, proliferation, and inflammatory cellular infiltration, as well as preventing an imbalance in the extracellular matrix composition. CO has also been associated with maintaining homeostasis of ions essential for normal cellular functions (e.g., Na+, Fe2+,3+). CO also targets cell-specific functions of the airway, such as reduction of contractility of airway smooth muscle cells, and preservation of the innate defense mechanism of airway epithelial cells. Further understanding of CO's effects on fundamental cellular functions in the airway will likely hold significant value in future considerations of CO's role in airway therapy and health.